1. Field of the Invention
The present invention relates generally to batteries and, more particularly, to managing the charging and discharging of a rechargeable battery.
2. Related Art
When a battery is charged, only one part of the supplied electric power is converted into charge. Another part of this power is converted into heat due to the internal resistance of the battery and, therefore, is lost for recharging. Such power loss can lead to an impermissible temperature rise of the host device. In those applications in which the battery is included in a component which is implanted in a patient, such temperature rise may also damage the surrounding tissue.
A further part of the supplied energy drives secondary electrochemical reactions which, for example, lead to gas evolution within the battery. This more commonly occurs when the battery has reached a higher charging level, for example, when the battery is charged to more than 80% of its nominal capacity. In particular, over years of operation, the capacity ratio of the positive and negative electrodes of a battery cell or cells shifts due to electrolyte loss and passivation and/or corrosion of the electrode surfaces. As a result, during charging (i.e., re-charging), a greater and greater preponderance of the gas-forming over the gas-consuming reactions occurs, and thus, the internal pressure of the cell rises quickly during charging. As the gas pressure rises, the cell housing swells, which under certain circumstances can lead to destruction of the cell or the device in which the cell is housed. The increasing corrosion and/or passivation of the electrodes and the concomitant decrease of the electrolyte-wetted electrode surface cause an increase of the internal resistance of the battery.
Thus, a charge management scheme is quite important in rechargeable batteries particularly if the batteries are to be used in an implantable device. Charge management regimes for rechargeable batteries are designed to maximize the useful life of the rechargeable battery by aligning their operating parameters with those empirically shown to be favorable for longevity. Furthermore, such charge management regimes seek to maximize the efficiency of the battery while addressing the adverse consequences associated with charging a battery.
The success of a charge management regime can depend on the accuracy of the input parameters used, such as the amount of charge remaining in the battery at any particular moment in time. One method of determining the charge remaining is to take measurements of the terminal voltage and use this to calculate accumulated charge.
However, voltage correlation techniques can be inaccurate due to variations in a battery's charging and discharging characteristics over its lifetime. These variations can be influenced by the manner in which the battery has been previously charged and discharged, and from the operating temperature. Clearly, this inaccuracy can lead to the undesirable consequences including those discussed above.
Another method of determining the charge remaining is to periodically sample the current flowing into or out of the battery. This method may generate a more accurate estimate of the present state of charge, and measurement gain or offset errors could be corrected using software signal processing techniques. To reduce quantization errors when sampling, the current resolution must be small relative to both charge and discharge currents. The main drawback to this method is its high processing requirement. This is particularly the case where the charge or discharge current is not constant over time. Such sampling needs to be at a rate of at least twice the highest component frequency (the Nyquist rate) of the current waveform, which may require a significant amount of processing power. In many electronic devices, this increased processing burden is impractical because of the limited space and power constraints.
Another method of determining the charge remaining is to mathematically integrate the measured charge and discharge current. The current integration approach has been used for example in U.S. Pat. No. 4,678,999 (“Schneider”). Similarly, U.S. Pat. No. 6,049,210 (“Hwang”) has highlighted the usefulness of frequency measurements derived from an integrator. U.S. Pat. No. 6,504,344 (“Adams”) demonstrates the integration technique used over a short period of time and under a known load to characterize the battery. However, a drawback of the current integration approach is that inaccuracies can result from the inherent offsets in analog circuitry. Compensation for these offsets has been shown to be more difficult to achieve for lower currents.
It is desired to ameliorate any one or more of the foregoing drawbacks of the above conventional techniques.